Projection screens of transmission type or reflection type are conventionally used for projecting television image or micro-film images. Usually, a projection screen has specific lens surfaces on its incident and/or viewing side in order to provide a brighter image and a greater viewing angle for the viewer of the screen. In particular, double-sided lenticular lenses are conventionally used for a projection screen. Commercially practical double-sided lenticular lenses are fabricated by various known methods such as extrusion, molding by a cell cast or heat-pressing. In these known methods, lenticular lenses are produced by transferring the pattern of a metal or resin mold directly or indirectly to a resin plate.
In a double-sided lens, the lenses on either side of the screen each has its own optical axis. The performance quality of the projection screen described above is seriously affected by any offset between these optical axes. Therefore, in the production of the projection screen it is essential that the optical axes of lenticular lenses on opposite sides of the screens are aligned with a high degree of precision. For instance, in the case of a projection screen having double-sided lenticular lenses with a lens pitch of approximately 1 mm, the tolerance of offset between the optical axes, as well as the variation in thickness, is as small as 2%, i.e., approximately 20 m.mu., in order to avoid degradation of color balance, restriction of the field of vision, color variation in the display frame, and so forth. Thus, the production of a projection screen having lenticular lenses on opposite sides essentially requires that the following conditions are met:
A: Molds for both sides of the lenticular lenses have a high degree of dimensional precision.
B: Mold temperature is uniform over the entire part of the mold so as to ensure uniform shrinkage of the mold during formation of the lenticular lenses as described above.
C: Both molds are precisely aligned with respect to each other to prevent loss of optical quality.
Conventional metals utilized for the production of lens molds have the following coefficient of thermal expansion values:
______________________________________ steel: 1.1 .times. 10.sup.-5 1/.degree.C. aluminum: 1.7 .times. 10.sup.-5 1/.degree.C. brass: 1.8 .times. 10.sup.-5 1/.degree.C. ______________________________________
Thus, steel, aluminum and brass exhibit a large thermal expansion or shrinkage of 11, 17 and 18 microns per meter of length in response to a 1.degree. C. temperature change. Consequently, it becomes necessary to precisely manufacture the mold, to accurately control the molding and to precisely position the molds. As a practical matter, however, it is extremely difficult to control the dimensions of the molds and the mold temperature with a high degrees of precision. For these reasons, it has been difficult to form lenticular lenses on opposite sides of a screen with a high degree of alignment accuracy.
Recently, there has been an increasing demand for a quality lenticular lenses in order to meet the current demand for high resolution large-screen television displays. In the known methods described above, however, it is difficult to obtain high quality lenticular lenses with an acceptable small lens pitch offset between both sides of the projection screen when the screen is approximately 1 square meter or larger. Thus, it has been impossible to meet the demand for the high quality lenticular lenses.
Under these circumstances, a method has been proposed in which cylindrical columnar transparent members are arranged side-by-side to form a double-sided lenticular lens sheet having no offset of the lens pitch between both projection side and viewing side of the screen.
However, this method suffers from a problem in that lights leak through clearances between adjacent cylindrical columnar members so as to cause undesirable effects such as see-through or hot band on the screen.
In order to alleviate this problem, another method has also been proposed for producing a projection screen in which a multiplicity of plastic strands are melt-extruded and arranged such that adjacent strands are fused together at their sides. This method is disclosed, for example, in the specifications of the U.S. patent application No. 07/441,385, European Patent Application No. 89121877.8, and Taiwanese Patent Application Nos. 78/109315 and 89/17296. However, the projection screen of the present invention is not specifically disclosed in these patents.
Recently, a method was proposed in which lights of three primary colors are superimposed onto a projection screen so as to form a color display. In this method, the optical axis of one of the three primary colors is positioned at a right angle to the screen, while the optical axes of two other colors are inclined with respect to the plane normal to the plane of the screen. This often causes a local color imbalance on the screen. Projection screens for color display are required to have excellent optical performance so as to avoid problems such as the local color imbalance.
Thus, none of the proposed methods have yielded satisfactory results.